French Patent No. 85-08836 (2 583 250) describes a technique for exciting plasma at the electronic cyclotron resonance. Such resonance is obtained for a magnetic field B and an excitation frequency f linked by the relation ##EQU1## in which m and e are the mass and charge of the electron. For example, for a frequency of 2.45 GHz, a magnetic field of 0.0875 Tesla is necessary to obtain the resonance.
The technique described in said French Patent Application requires the use of permanent magnets, each one of which creates, near its pole, at least one surface having a constant magnetic field and of intensity corresponding to the electronic cyclotron resonance. The electromagnetic energy is brought to the level of the resonance zone via antennas or plasma exciters, each one constituted by a metal wire element. Each exciter is placed in vertical relationship to the permanent magnets which are mounted on the wall of the enclosure.
The magnetic field of intensity equal to the value giving the resonance, and the electromagnetic field are both situated and confined essentially in the space existing between an exciter and the part of the enclosure wall placed in facing relationship to a magnet. In the presence of a gaseous medium of reduced pressure, the electrons are accelerated in the resonance zone and wind around the lines of magnetic field which define a surface for confining the plasma. Said field lines, which are scallop-shaped, join the pole of one magnet to its opposite pole or to the opposite pole of the next magnet. Along its path, the electron dissociates and ionizes the molecules and the atoms with which it collides. Then, the plasma which has thus formed in the scallops of magnetic field diffuses from said field lines in order to form a cold plasma virtually free of energetic electrons, these remaining trapped in the scallops.
With no gas in the enclosure, the microwave energy propagates without any loss along the exciter constituted by the wire applicator positioned close to the poles of the permanent magnets. With a gas to be ionized in the enclosure, the plasma is excited with the electronic cyclotron resonance and therefore there has to be losses when the microwaves propagate and the available microwave power reduces gradually from the microwave source along the exciter. It is also found that the plasma density reduces at the same time as the microwave power.
Another disadvantage of the above-described technique resides in the fact that the microwave energy can only be applied to one of the ends of the wire exciter. As a result, the microwave energy cannot be applied to a looped antenna, such as a circular antenna.